The invention relates to the field of fiber optic sensors and modulators. More particularly, the invention relates to the field of frequency shifters for fiber optic systems.
It has long been known that light from a moving source will be shifted in frequency because of the Doppler effect. The change in frequency or wavelength is a function of the relative velocity of the source and observer. The wavelength will become shorter and the frequency high when the source is moving toward the observer, and vice versa.
It has also been known that the wavelength of light will be changed upon reflection from a moving mirror. The moving mirror adds to the energy content of the impinging photon thereby increasing the frequency of the light. This phenomenon is discussed by Meyer-Arendt in "Introduction to Classical and Modern Optics", Prentice Hall (1972) at pages 539-540.
The Doppler effect principle and hetrodyning effects have been used in bulk optics to cause frequency shifts in light waves reflected from wavefronts of acoustic waves propagating through optically transparent bulk media. The areas of compression and rarification caused by the travelling acoustic wave change the index of refraction in the bulk media such that incoming light impinging obliquely on the wavefronts is partially reflected and partially refracted. The movement of the wavefronts causes a Doppler shift in the reflected and refracted light similar to the effect of a moving mirror.
A single sideband modulator for producing phase, or frequency shift in integrated optic waveguides was taught by Heisman & Ulrich in "Integrated Optical Single Sideband Modulator and Phase Shifter", IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, April 1982 at pp. 767-771. A scheme of spatially weighted coupling points between two waveguide modes was mathematically proposed, and a physical implementation was taught for a bulk optic strip waveguide diffused into an X-cut LiNbO.sub.3 waveguide. The coupling between two modes in this birefringent crystal was implemented by use of pairs of interdigital electrodes spaced at 1/4 of the beat length. Each electrode had a width of 1/4 beat length or an integer multiple thereof. The electric fields under the edges of the electrodes caused coupling by the electro-optic effect found in crystals. The electrodes were driven by driving voltages which were 90 degrees out of phase to simulate a travelling wave of off diagonal polarizability. The frequency shift was caused by forward light scattering at the moving perturbation comparable to Bragg reflection at a travelling acoustic wave.
The integrated optic device described above has the advantage that the amount of frequency shift is limited only by the upper frequency of the driving signals. However, it has the extreme disadvantage that it cannot be easily used in fiber optic systems because of the complications of aligning and coupling the integrated optic waveguide to the fiber of the host system. Such difficulties render the device undesirable for use in fiber optic systems where in-line devices fabricated on the fiber of the host system alleviate the need for complicated coupling apparatus which is troublesome to install and properly align.
Further, integrated optic devices are very lossy by the nature of their construction. Integrated optic waveguides are made by diffusing impurities such as titanium into single crystal structures to form a strip. These diffused waveguides are lossy for several reasons. First, the presence of impurities causes absorption and scattering losses. These losses are on the order of decibels per centimeter. Further, integrated optic waveguides are subject to an index changing phenomenon called the "photorefractive effect". The effect stems from the fact that when a large amount of optical power is concentrated in a small area of a crystal, the optical electromagnetic field becomes so strong that it pushes electrons in the crystal structure away from the waveguide area. This causes the index of the waveguide to vary such that the waveguide is no longer monomode thereby rendering the device inoperative.
A further disadvantage of integrated optic waveguides is that they are difficult to make. Monomode waveguides have cores on the order of 10 microns or less in diameter. Because diffusion of impurities into a crystal is necessary, and the geometries are very small, complex integrated circuit techniques must be used. First, a mask must be made, and then complicated and expensive equipment must be used to lay down layers of photo-resist and to diffuse the impurities into the crystal. Further, precise registration of the mask must be maintained to insure the correct alignment for the electrode fingers and the diffused waveguide. These additional complications render integrated optic devices undesirable for use in fiber optic systems.
An in-line acousto-optic frequency shifter was taught by Nosu, et al in "Acousto-Optic Frequency Shifter For Single Mode Fibers" first published at the 47th International Conference on Integrated Optics and Optical Fiber Communications Conference in Tokyo, June 27-30, 1983. A birefringent monomode fiber was mounted in two oil filled PZT cylinders with their respective leading edges spaced 3/4 of a beat length apart. PZT, as is well known to those skilled in the art, changes its dimensions when an electric field is applied to it. The fiber was placed in a capillary tube filled with mineral oil, and the capillary tube was placed in the PZT cylinder in an off-axis position. The PZT cylinder was filled with mineral oil. A standing pressure wave in each cylinder resulted when the PZT cylinders were excited with sinusoidal excitation signals phased 90.degree. apart causing elasto-optic coupling. This elasto-optic coupling between the polarization modes of the fiber in one cavity caused sidebands above and below the optical carrier. The other cavity generated one sideband that was in phase and another that was out of phase with the sidebands created by the first cavity such that one sideband was strengthened and the other was cancelled.
The frequency shift in the Nosu et al device above was caused by the excitation of moving acoustic waves in the fiber by the PZT cylinders. Each cylinder established two acoustic waves moving in opposite directions. The opposite directions of travel caused the upper and lower sidebands to occur.
Because the PZT oil filled chambers were mechanically weakly coupled to the fiber, not much power was transferred between modes by the two drums. Further, the drums were huge, rendering the device too large for effective use in many practical fiber optic devices. If enough of these drums were used such that a large amount of power was transferred between the two modes, the resulting device would be quite unwieldy and generally impractical for use in a fiber optic system.
Accordingly, a need has arisen for a fiber optic frequency shifter which is compact in size, and which can be fabricated on the fiber of a monomode, fiber optic system. The system should be able to couple from 0 to 100% of the input power from one mode to another mode at a shifted frequency which is exactly equal to the modulating signal with as few harmonics as possible.